The present invention realtes to graphite materials and, more specifically, to graphite structures of light weight and excellent elasticity and to a method for the production of such structures.
Generally available carbon materials, whether carbonaceous or graphitic, are characterized in that they are rigid structures and possess high Young's modulus. Light-weight carbon materials, on the other hand, include carbon foams, hollow carbon spheres and expandable graphite.
Carbon foams have been prepared either by foaming, curing and calcining polyurethane or phenol resins or by forming and calcining hollow carbon spheres with the aid of a binder (see U.S. Pat. Nos. 3121050, 3342555 and 3302999, and Inada, et al., "Carbon", No. 69, page 36, 1972). Such foams are found to have a bulk density of the order of about 0.5 g/cm.sup.3, but their graphitized structures have poor flexibility and are thus rigid.
Hollow carbon spheres have been produced by the melting and atomizing of foam-containing pitches into spherical form, followed by calcination (see Amagi, "Materials", Vol. 16, page 325, 1971). Such spheres are relatively light-weight materials, as expressed in terms of bulk density of 0.1 to 0.3 g/cm.sup.3, but are rigid for lack of flexibility.
Expandable graphite has generally been made by the oxidation and heat-treatment of naturally occurring scaly graphite. This graphite is light in weight as expressed in terms of its coefficient of expansion which may reach a factor of several hundreds, but may be subjected to compression molding, as will be appreciated from the fact that it is usable as the starting material for graphite sheets. Graphite sheets obtainable from such an expandable graphite are flexible and possess elasticity to such an extent that they are restorable to their original form after a compression load has been applied thereto and removed therefrom. For that reason, they are said to excel in air-tightness when used as packing material. However, such sheets are of a densified small as about 40% and a recovery of as small as about 20%, when they are subjected to a compression load of 350 kg/cm.sup.2 [Saito, "Kogyo Zairyo" ("Industrial Materials"), Vol. 20, page 34, 1985].
On the other hand, mesocarbon microbeads obtained by the separation of minute mesophase-spheres formed at the incipient stage of carbonization of pitches are one form of carbonaceous mesophases. In one method proposed in Japanese Patent Laid-Open Publicaion No. 60(1985)1508319, a microporous carbonaceous material is obtained by nitrating and heat-treating such microbeads. However, this method produces only microporous structures by that heat treatment without giving rise to any substantial icnrease in volume, and is not directed to reductions in weight.